Electric furnace operation method

Abstract

A method for operating an electric furnace is provided, in which, when a raw metallic material is melted in the electric furnace to produce a molten metal, high productivity is obtained and the refractory wear is less. The method for operating an electric furnace comprises stacking and accommodating, inside the electric furnace, a molten metal, slag, a raw metallic material, and a subsidiary raw material in this order and continuously discharging a molten metal and slag, wherein: the electric furnace comprises a bottom part, furnace walls, a furnace lid, and a means for molten-metal stirring; and the input power per surface of the molten metal is 300 kW / m2 or greater and the density of the power of stirring the molten metal by the stirring means is in the range of 5-100 W / t.

Description

[Technical Field]

[0001] The present invention relates to an electric furnace operation method for producing molten metal by melting a metal raw material containing reduced iron in an electric furnace, particularly a submerged arc furnace. In the following description, the unit of mass "t" is 10 3 The unit "L" represents kilograms and is a unit of volume. -3 m 3 This represents the standard conditions of 0°C and 101325 Pa, indicated by "N" before the unit of gas volume. In this specification, "x~y" representing a numerical range means x or greater and y or less, including boundary values. [Background technology]

[0002] In recent years, there has been a growing demand to reduce CO2 emissions in order to lessen the environmental burden. In the steel industry, the direct reduction (DR) method for producing iron sources is attracting attention as an alternative to the blast furnace method, which emits a large amount of CO2. In the DR method, for example, iron-containing agglomerates are reduced in a shaft furnace to produce directly reduced iron (DRI). This reduced iron and other iron source materials such as iron scrap are charged into an electric arc furnace (EAF) or submerged arc furnace (SAF) and heated and melted, and molten iron is produced after separating the slag.

[0003] In Japan, studies are underway to apply the DR (Dry Reduction) method to low-grade iron ore produced in Australia, India, and other countries. Currently, it is difficult to melt and process reduced iron produced from low-grade iron ore using the EAF (Earth-Axial Fountain) method. Therefore, the use of a submerged arc furnace (SAF) is being considered.

[0004] The production of molten iron by SAF is a process in which metallic materials and auxiliary materials such as scrap, pig iron, directly reduced iron (DRI), molten iron, or hot briquetteed iron (HBI) are continuously supplied into a submerged arc furnace (SAF) without opening the furnace lid, or charged in small buckets. This SAF melting process may also be carried out in flat-bath operation. During the charging of raw materials, electrodes are inserted into the molten slag in the submerged arc furnace, and arcs are generated between multiple electrodes or between electrodes and molten iron. An electric current is also passed between the raw materials and the slag, heating and melting the raw materials through resistance heating and heat transfer.

[0005] One problem with the SAF melting process is the inefficient heat transfer to raw materials located far from the electrodes. This leads to delayed melting of the metal raw materials, causing problems such as concentration gradients, unreliable temperature measurements, unreliable process control, and over-temperature molten metal tapping. To solve this non-uniform temperature problem, electromagnetic stirring disclosed in Patent Document 1 and gas stirring disclosed in Patent Document 2 have been developed. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Special Publication No. 2020-505579 [Patent Document 2] Japanese Patent Publication No. 2002-317918 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, the conventional technology had the following problems. Specifically, the technology described in Patent Document 1 is a technology applied to so-called electric arc furnaces. In electric arc furnaces, the slag ratio to molten metal is low and the Fe content is high during operation. It cannot be directly applied to a technology in which molten metal, slag, metal raw materials and auxiliary materials are sequentially layered and contained in the electric furnace, and the molten metal and slag are continuously discharged and the operation is carried out in a reducing atmosphere. Furthermore, the technology described in Patent Document 2 is an electric furnace for waste melting furnaces and is not intended for the mass production of molten metal. The challenge has been to develop an electric furnace operation method that can be applied to electric furnaces, especially submerged arc furnaces, and that takes into account the stable production of molten metal.

[0008] This invention has been made in view of the above circumstances, and aims to provide an electric furnace operation method that is productive and minimizes refractory wear when melting metal raw materials in an electric furnace to produce molten metal. [Means for solving the problem]

[0009] The present invention provides an electric furnace operation method that advantageously solves the above problems, comprising: a method for operating an electric furnace in which molten metal, slag, metal raw materials and auxiliary materials are sequentially stacked and contained in the electric furnace, and the molten metal and slag are continuously discharged, wherein the electric furnace comprises a bottom, furnace walls, furnace lid, and means for stirring the molten metal, and the input power per surface of the molten metal is 300 kW / m². 2 The above is characterized in that the stirring power density of the molten metal by the stirring means is in the range of 5 to 100 W / t.

[0010] Furthermore, the operating method of the electric furnace according to the present invention is as follows: (a) The electric furnace shall be an electric furnace capable of melting without intentionally supplying oxygen gas, (b) The stirring means is an electromagnetic stirrer provided at the bottom of the electric furnace. (c) The stirring means is a gas blowing from an immersion lance or a gas bottom blowing means provided at the bottom of the electric furnace. (d) The discharge port of the molten metal and the discharge port of the slag are provided on the furnace walls in different directions with respect to the center of the furnace bottom. By penetrating the plugging material from the discharge port filled with the plugging material and opening, the molten metal is discharged. (e) A part or all of the metal raw material is reduced iron. (f) The basicity of the slag is in the range of 1.0 to 1.5. Here, the basicity of the slag is the ratio of CaO to SiO2 in the slag on a mass basis. (g) An arc furnace is used for the electric furnace, and at least one of the electrode height, the arc voltage, and the arc current is adjusted so that the value E / √I obtained by dividing the arc voltage E (V), which is the voltage to the ground, by the square root of the current I (A) per arc electrode is 2.0 or less. (h) The electric furnace is a submerged arc furnace. etc. can be more preferable solutions.

Advantages of the Invention

[0011] According to the operation method of the electric furnace according to the present invention, by appropriately stirring the molten metal, a sufficient input power can be loaded, the temperature rise of the molten metal can be suppressed, the wear of the refractory can be suppressed, and the molten metal can be produced with good productivity.

Brief Description of the Drawings

[0012] [Figure 1] It is a schematic longitudinal sectional view for explaining the operation method of the electric furnace according to an embodiment of the present invention. (a) shows an example having electromagnetic stirring equipment, and (b) shows an example of bottom blowing stirring. [Figure 2] It is a graph showing the influence of the stirring power density of hot metal on the relationship between the input power per unit surface area of the hot metal in the electric furnace and the hot metal temperature. [Figure 3] It is a schematic longitudinal sectional view showing a submerged arc furnace without a stirring function for molten metal.

Modes for Carrying Out the Invention

[0013] Hereinafter, embodiments of the present invention will be specifically described. The following embodiments illustrate facilities and methods for embodying the technical idea of the present invention, and do not specify the configuration as follows. That is, the technical idea of the present invention can be variously modified within the technical scope described in the claims.

[0014] FIG. 1 is a schematic longitudinal sectional view for explaining the configuration of a submerged arc furnace suitable for use in an operation method of an electric furnace according to an embodiment of the present invention. FIG. 1(a) is an example provided with an electromagnetic stirrer 6 composed of a linear motor. FIG. 1(b) is an example in which gas is blown from the bottom blowing tuyere 7 to blow bubbles 7A. The submerged arc furnace 1 stores a molten iron P, a molten slag (slag) S, and a raw material M composed of a metal raw material and a subsidiary raw material in a stacked manner within a furnace wall 2. In the submerged arc furnace 1, an electrode 3 is inserted into the molten slag S, and an arc is generated between a plurality of electrodes 3 or between the electrode 3 and the molten iron P, and the raw material is heated and melted by radiant heating or by resistance heating of the current flowing between the electrode, the raw material, the slag, and the molten iron. In the present embodiment, electric heating is performed in a so-called reducing atmosphere without intentionally supplying oxygen. "Not intentionally supplying oxygen" does not prevent the mixing of air from the charging of the raw material, the blowing gas, the gaps in the refractory, etc. It means not using the heat of oxidation for heating the raw material.

[0015] In the present embodiment, it is preferable to charge the raw material 4 from above and continuously produce molten metal. In that case, it is preferable to have a discharge port for the molten metal and a discharge port for the slag, and to arrange them in different directions with respect to the center of the furnace bottom. It is preferable to fill a closing material in each discharge port and open it by penetrating the closing material when a predetermined amount of molten metal can be held, thereby discharging the molten metal.

[0016] In this embodiment, an iron source raw material containing reduced iron is charged into a submerged arc furnace 1. Along with this, at least one or both of the following are added as auxiliary raw materials: a slag-forming agent to adjust the basicity (CaO / SiO2) of the molten slag S formed on the molten iron P, and a carbon material to adjust the carbon content of the molten iron P. The basicity (CaO / SiO2) of the molten slag refers to the ratio of CaO to SiO2 in the molten slag by mass fraction.

[0017] In this embodiment, the iron source material melted in the submerged arc furnace 1 includes reduced iron. The reduced iron may be produced in advance by a direct reduction method, or commercially available reduced iron may be purchased and used. Iron scrap or scale can also be used as the iron source material. The mass ratio of reduced iron to the iron source material is preferably 50 to 100%.

[0018] A metallization rate of 60% or higher for reduced iron results in a lower power consumption per unit area and excellent energy efficiency. There is no particular upper limit to the metallization rate. If the metallization rate is too high, in the submerged arc furnace 1, the reduced iron, which is the raw material 4 introduced from above the molten slag S, may conduct electricity within the slag, potentially reducing the heat of resistance. Therefore, it is preferable to set the upper limit of the metallization rate to around 90%.

[0019] It is preferable to add limestone (CaCO3) or quicklime (CaO) as a CaO source and silica (SiO2) as an SiO2 source to the slag-forming material. It is preferable to use it in the form of lumps or powders, together with or mixed with metal raw materials, for example, by granulation. It is preferable to set the basicity (CaO / SiO2) of the molten slag S to the range of 1.0 to 1.5. When the basicity of the molten slag is within this range, the composition is similar to that of blast furnace slag, making it suitable for reuse as roadbed material such as cement. It is also preferable that the total iron content in the molten slag be 5.0% by mass or less. This can be achieved by intentionally not supplying oxygen gas, and the iron yield is improved.

[0020] The carbon material can be coke or coal. Biomass charcoal may also be used. It is preferable to use it in lump form or powder form, together with or mixed with metal raw materials, for example, by granulation. The carbon content in molten iron P is preferably adjusted to a range of 2.0 to 5% by mass. Within this range, it is suitable for use as pig iron, either as is or as a raw material for steelmaking in the next process.

[0021] In this embodiment, electrical energy is used to melt the metal raw material, for example, reduced iron. Therefore, a reduction in CO2 emissions can be obtained compared to cases that use the heat of carbon combustion, such as the blast furnace method. It is preferable to use electrical energy generated from renewable energy sources.

[0022] If an electric furnace operation method for producing molten metal in the same manner as in this embodiment is used with a submerged arc furnace that does not have a molten metal stirring function, as shown in Figure 3, the following problems arise. In such a submerged arc furnace, there is no external oxygen supply and no solid-liquid flow occurs within the furnace, so heat transfer from the molten metal P to the raw material M via the molten slag S is almost entirely by heat conduction. Also, the electrode 3 is usually positioned in the center when viewed from above. Therefore, the heat supply to the raw material M is limited, and even if the input power is increased, most of the input power is consumed in heating the molten metal P, and the input power used for melting the raw material M, especially the heat supply to the raw material M at positions far from the electrode 3, is reduced. Power density per molten iron surface is 250 kW / m³ 2 At this temperature, the molten iron temperature exceeds 1500°C. Since the rate of wear of typical refractories increases sharply above 1550°C, the refractories will be severely worn down under the above conditions. Therefore, a typical submerged arc furnace has a power density of 250 kW / m³ per molten iron surface. 2 In many cases, furnaces are operated at a certain level. To produce large quantities of molten metal using such furnaces requires large-area equipment and numerous pieces of equipment, resulting in excessive investment. Therefore, after investigation, the inventors found the relationship between the stirring power density ε and the molten iron temperature shown in Figure 2, making it possible to apply a high power density while suppressing the molten iron temperature.

[0023] As a result of intensive studies, the inventors have found a relationship that the relationship between the power density per unit surface area of hot metal and the temperature of molten metal P depends on the stirring power. The results of the study are shown in FIG. 2. For example, in a normal submerged arc furnace, the stirring power density ε = 0, and it can also be explained that in a conventional submerged arc furnace, it is operated at about 250 kW / m 2 Furthermore, the inventors newly discovered that by applying even a small amount of stirring power, a large amount of power can be input into the furnace without increasing the temperature of the molten metal P.

[0024] The submerged arc furnace 1 of this embodiment includes a bottom, a furnace wall 2, a furnace lid (not shown), and a stirring means for molten metal. The input power per unit surface area of the molten metal is set to 300 kW / m 2 or more, and by setting the stirring power density ε of the molten metal by the stirring means in the range of 5 to 100 W / t, high productivity and stable operation can be achieved without excessive capital investment. The input power per unit surface area of the molten metal is preferably in the range of 500 to 2500 kW / m 2 The stirring power density ε is preferably in the range of 10 to 30 W / t.

[0025] If the stirring power density is less than the lower limit, there is a risk that the wear of refractories such as the furnace wall will progress due to overheating of the molten metal. If the stirring power density exceeds the upper limit, the flow rate of the molten metal may be too fast and the wear of the refractory may progress.

[0026] As an example of a means of stirring molten metal, it is preferable to install an electromagnetic stirrer 6, as shown in Figure 1(a), at the bottom. It is preferable that the bottom on which the electromagnetic stirrer 6 is installed is made of a non-magnetic material. The stirring power density by electromagnetic stirring can be determined, for example, by numerical analysis or experimentally, relative to the output of the electromagnetic stirring. For example, it can be about 0.5% of the output power of the electromagnetic stirring. The electromagnetic stirrer 6 can be made of a linear motor, for example, and can generate a traveling magnetic field in the direction from the center of the furnace bottom toward the furnace wall or from the furnace wall toward the center of the furnace bottom. Alternatively, the electromagnetic stirrers may be arranged opposite each other across the center of the furnace bottom to generate traveling magnetic fields in different directions, or the electromagnetic stirrers may be arranged circumferentially to generate a swirling flow in the molten metal. The flow of molten metal due to electromagnetic stirring will, for example, flow along the bottom of the furnace, reverse direction at the furnace wall, and generate heat transfer on the surface of the molten metal based on forced convection.

[0027] Another example of a means of stirring the molten metal is to install a bottom-blowing tuyere 7, as shown in Figure 1(b), at the bottom of the furnace and blow gas into it to stir with bubbles 7A. Instead of a bottom-blowing tuyere, gas may be blown from the tip of an immersion lance. The stirring power density can be calculated from the flow rate of the blown gas, for example, by the following equation (1).

[0028] ε=(371×Q×Tm / Wm)×[ln{1+(9.8×ρm×h / P)}+(1-Tg / Tm)] (1) However, ε: stirring power density (W / t), Q: Total gas flow rate (Nm³) 3 / sec), Tm: Molten metal temperature (K), Wm: Weight of molten metal (t) ρm: Molten steel density (t / m 3 ), h: For top-blowing gas, this is the distance (m) from the molten metal surface at the tip of the lance; for bottom-blowing gas, this is the distance (m) from the molten metal surface to the bottom of the ladle. P: Atmospheric pressure (101.325 kPa) Tg: Gas temperature (K)

[0029] In the above embodiment, an electric furnace that is roughly circular when viewed from above was used as an example, but the shape is not limited to elliptical or rectangular.

[0030] In this embodiment, the electric furnace 1 is preferably an arc furnace. In the submerged arc furnace preferably used in this embodiment, current flows between the electrode, raw material, slag, and molten iron, so the voltage to ground at the same current value is generally lower compared to electric furnaces that generate an arc in air. This is because the electrical conductivity of slag is significantly higher than that of air. Therefore, it is preferable to adjust at least one of the electrode height, arc voltage, and arc current so that the value E / √I, obtained by dividing the arc voltage E(V), which is the voltage to ground, by the square root of the current I(A) per arc electrode, is 2.0 or less. More preferably, the index E / √I should be 1.5 or less. [Examples]

[0031] The investigation was conducted using a submerged arc furnace 1 with a scale of 4-6 tons, as shown in Figure 1. Table 1 summarizes the specifications of the three-phase AC electric furnace used in the investigation.

[0032] [Table 1]

[0033] In submerged arc furnace 1, shredder scraps, carbon, and blast furnace slag were charged and power was applied for initial melting. The pre-charged amounts were adjusted to produce 750 kg of molten iron and 1500 kg of slag. After the pre-charged materials had completely melted, reduced iron as the main raw material and pre-mixed raw materials such as carbon, lime, alumina, and MgO sources to adjust the carbon concentration in the molten iron and the composition of the slag were continuously fed in. The maximum amount of molten iron was targeted at 4 tons, and this was adjusted by the amount of raw materials fed in. The final thickness of the molten iron was 140 mm.

[0034] The input power of the submerged arc electric furnace was adjusted by tap voltage and electrode position to ensure the power met the predetermined conditions.

[0035] The transformer used for the electromagnetic stirring device was 200 kVA, and its output was adjusted. The power factor at maximum output was approximately 50%, and a maximum output of approximately 100 kW was possible. Since it is difficult to measure the contribution rate of electromagnetic stirring directly, it was calculated in advance using a numerical analysis method and set to 0.5%. This value is typical for electromagnetic stirring devices and does not change significantly. The operating conditions are shown in Table 2.

[0036] For bottom-blowing agitation, a porous nozzle was installed and agitation was performed using N2 gas. The agitation power density was calculated using the above equation (1), and the bath density was set to 7.0 t / m³. 3 The calculations were performed assuming a molten metal height h of 0.14m, a gas temperature Tg of 300K, and a molten metal temperature Tm of 1800K. The operating conditions are shown in Table 3.

[0037] In the evaluation column, the amount of refractory material loss (mm) was evaluated for each level, and the amount of loss under the conditions of Test No. 1 was standardized to 1. Anything more than twice that amount was evaluated as ×. Productivity was evaluated by evaluating the time from when reduced iron was first added until all raw materials had melted. Anything that could not produce 4 tons of molten iron within 100 minutes was evaluated as ×. All other results were evaluated as ○. The results are shown in Tables 2 and 3. The values ​​of the operational index E / √I are also shown in Tables 2 and 3.

[0038] [Table 2]

[0039] [Table 3]

[0040] Input power per surface of molten metal: 300 kW / m² 2 In the above-described example, where the stirring power density of the molten metal by the stirring means was in the range of 5 to 100 W / t, both productivity and refractory wear were excellent. Comparative examples that deviated from these conditions were inferior in either productivity or refractory wear. [Explanation of Symbols]

[0041] 1. Electric furnace (submerged arc furnace, SAF) 2 Furnace wall 3 electrodes 4 (Input) Raw materials 5 Arc 6. Electromagnetic stirrer (linear motor) 7 Bottom-blown tuyere 7A Air bubbles P Molten iron (molten metal) S Molten slag (molten residue) M Raw material F (flow of molten metal)

Claims

1. A method for operating an electric furnace, which is a submerged arc furnace, wherein molten metal, slag, metal raw materials and auxiliary materials are sequentially stacked and contained within the electric furnace, and the molten metal and slag are continuously discharged during operation, The electric furnace comprises a bottom, a furnace wall, a furnace lid, and a means for stirring the molten metal. Input power per surface of molten metal: 300 kW / m ２ That concludes my presentation. A method for operating an electric furnace, wherein the stirring power density of the molten metal by the stirring means is in the range of 5 to 100 W / t.

2. The method for operating an electric furnace according to claim 1, wherein the electric furnace is an electric furnace capable of melting without intentionally supplying oxygen gas.

3. The method for operating an electric furnace according to claim 1, wherein the stirring means is an electromagnetic stirrer provided at the bottom of the electric furnace.

4. The method for operating an electric furnace according to claim 1, wherein the stirring means is a gas blowing from an immersion lance or a gas blowing means provided at the bottom of the electric furnace.

5. The molten metal outlet and the slag outlet are located on the furnace walls in different directions relative to the center of the furnace bottom. A method for operating an electric furnace according to claim 1, wherein molten metal is discharged by opening an outlet through which a blockage material has been packed, thereby penetrating the blockage material.

6. The method for operating an electric furnace according to claim 1, wherein part or all of the metal raw materials are reduced iron.

7. The basicity of the slag is in the range of 1.0 to 1.5, where the basicity of the slag is the amount of SiO in the slag on a mass basis. ２ The method for operating an electric furnace according to claim 5, wherein the ratio of CaO to is the ratio of the other to the other.

8. The method for operating an electric furnace according to Claim 1, wherein at least one of the electrode height, arc voltage, and arc current is adjusted so that the value E / √I obtained by dividing the arc voltage E (V), which is the voltage to ground, by the square root of the current I (A) per arc electrode is 2.0 or less.

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